1. Field of the Invention
The present invention relates to an epoxy resin composition, and more particularly, to an injection-moldable epoxy resin composition which is excellent in thermal stability without being cured in a cylinder of an injection-molding machine and is extremely suppressed in the formation of flux on molding.
2. Description of the Related Art
It is well known that epoxy resins are excellent in various physical properties such as heat-resisting property, chemicals-resisting property and insulating property and are employed for various uses.
From the past, a transfer molding method is known as a means for molding epoxy resin materials which is appropriate in the aspect of productivity. In transfer molding, it is ordinary that an epoxy resin composition is cast in a pot of a metal mold prior to molding and the composition is then pressed by a plunger and cured under fluidization within the metal mold.
In this molding method, however, a material for molding has initially to be shaped into tablets and a preheating step is needed so that shortening of the molding cycle becomes naturally limitative, thus making producibility and economization of cost difficult to be attained. In transfer molding, moreover, there remains such a problem in the course of molding that a cull part remaining in the pot in addition to a runner part becomes a waste portion after curing.
The cured cull part is normally discarded as a waste without being recycled, but the waste is not decomposed by biodegradation and thus causes an environmental pollution.
In an injection-molding method, on the other hand, a thermoplastic resin powder for molding is only charged into a hopper whereby the subsequent preheating and weighing operations of the powder are automatically operated in a cylinder and the fluidized resin is injected into a metal mold where curing of the resin is effected. In this case, therefore, steps for shaping the molding material into tablets and preheating them are not required and furthermore a continuous production of the moldings becomes possible, thus making shortening of the molding cycle possible as compared with the transfer molding operation. The injection-molding method can thus be said to be superior in the aspect of production efficiency.
However, epoxy resin molding materials employed heretofore in transfer molding are inferior in thermal stability in the cylinder so that melt viscosity of the materials increases extremely to make injection-molding impossible.